Hybrid metal packages have been used for many years to hermetically protect hybrid and semiconductor discrete and integrated circuit chips. The chips are mounted within the eyelet or body of the package and are electrically connected to external circuitry by means of conductive leads or pins passed through apertures in the body. To ensure hermetic sealing and to preclude short circuiting between the lead or pins and the metallic package, the leads or pins are sealed in glass and the glass-pin combination is sealed in the body.
The formation of hermetic compression type glass-to-metal seals as exemplified in U.S. Pat. Nos. 3,370,874, 3,600,017, and 4,008,945 utilize glass and metal components possessing intentionally mismatched coefficients of thermal expansion (CTE) so that as thermal energy is applied and removed during seal formation and during subsequent operational use, the glass and metal components expand and contract, respectively, at different rates. Hermeticity in glass-to-metal compression seals is achieved, without bonding, by the compressive forces or physical stresses created between the components by the dissimilar rates of contraction and expansion.
In practice, compression seals formed between stainless steels and suitable "compression" type glasses may be achieved with a compliant metal such as nickel or copper electroplated or sputtered onto the surface of the stainless steel to form a metallic pad or film. A thin plated-on metallic pad is considered beneficial in reducing glass cracking tendencies, although the fact that stainless steel, iron, nickel-iron, or low carbon steel are often compression sealed without the aid of intermediate metal films indicates that such procedures are not absolutely essential. Such films are more likely to be used in the event that the sealed metal is composed of nickel-iron, or iron where corrosion is a consideration.
Compression type glass-to-metal seals are manufactured using a wide range of glass components and metal bases. For example, hybrid packages fabricated from soft steel and pins of nickel-iron alloys such as number 52 alloy are often sealed using a Corning type 9008 or other equivalent "soft" glasses.
Many more vitreous or soft glasses are available for compression glass-to-metal sealing than the devitrified or hard type glasses which are utilized in matched glass-to-metal sealing. A common distinction between hard and soft glasses is that glasses having low coefficients of thermal expansion and contraction are considered "hard", while those having high coefficients of thermal expansion/contraction are considered "soft". There is some degree of overlap in the designation of glasses, however, and such notables as U.H. Partridge have referred to certain "low" CTE glasses of the borosilicate type as being soft. A language constraint between softness and toughness is a major cause of such ambiguity.
Hermeticity in matched glass-to-metal seals, in contrast, is achieved by molecular bonding between the glass and metal components. The surfaces of the metal components which interface with the glass are preconditioned prior to the sealing operation by the controlled growth of munsel-grey oxide on the interfacing surfaces of the metallic components. Sealing is subsequently effected by applying thermal energy to partially fluidize the glass so that it wets or flows over the oxide. As thermal energy is withdrawn from the heated components, i.e., cooldown, molecular bonding occurs between the glass and oxide. The oxide is mutually soluble in both the glass and metal components. The metallic oxide, metal and glass components forming the stabilized glass-to-metal seal have substantially matching CTEs over a wide temperature range in matched seals.
Glass-to-metal compression seals, in contrast, do not require preoxidation of the metal elements comprising the seal because molecular bonding is not required to effect a hermetic seal. Rather, hermeticity is achieved in glass-to-metal compression seals due to the physical stresses created by the mismatch in CTEs of the metal body, the glass inserts, and the metallic electrical conductors. The metal body or outer member has the highest CTE, the glass insert has an intermediate CTE and the conductor has the lowest CTE such that during cooldown after firing the body contracts in on the glass insert at a faster rate than the glass insert is contracting in on the conductor. The mismatch in CTEs between the body-glass insert and the glass insert-conductor creates tremendous compressive forces therebetween with the result that hermeticity is achieved without bonding.
FIG. 1 exemplifies a compression type glass-to-metal 10 seal of the prior art wherein a soft glass preform 12 is sealed to a nichel-iron alloy such as number 52 alloy terminal pin or lead 14 along interfacing surface 16 and to a stainless steel or soft steel alloy body 18 along interfacing surface 20. Positive menisci 22, 24 are formed at the ends of the interfaces 16, 20, respectively, i.e., at the boundary of the interfaces with the environment.
A large percentage of hybrid metal packages find end use in both high-tech industrial and governmental applications. It is, therefore, more efficacious, as a practical manner, to subject hybrid metal packages to quality control (QC) acceptance testing using QC standards meeting or exceeding government QC standards, rather then segregating hybrid metal package lots according to end use and then QC testing using different criteria. Not only would the latter procedure increase the overall production time and cost, necessitating for example tighter package lot control and segregation and recalibration or duplication of QC gear, but it would also vitiate the fungibility of finished metal packages.
Hybrid metal packages are generally subjected to four broad areas of QC testing: visual/mechanical; electrical; environmental; and line. Hybrid packages subjected to mechanical forces such as acceleration, shock or vibration, either during QC testing or in field use, have been found to experience a certain degradation in physical integrity.
Due to the partial fluidization and resolidification of the boundary layers of glass at the interfaces 16, 20 and at environmentally-exposed surfaces during the firing process, compressive skin stresses are set up in the outer layers of the glass preform 12. Exacerbating this condition are the inherent compressive stresses existing in the glass 12 and metallic elements 14, 18 due to the mismatch in CTEs. When subjected to mechanical forces glass-to-metal compression seals 10 of the prior art have been found to be adversely affected by crack or fracture formation and glass chip-out.
Of particular concern are the glass menisci 22, 24 formed during the sealing operation, these menisci being brittle and of relatively low toughness, strength and ductility. The menisci 22, 24 are subject to the highest concentration of compressive skin stresses due to their structural configuration. The meniscus 22, formed at the interface 16 between the glass 12 and the terminal pin 14, is especially vulnerable since most of the mechanical forces experienced by the hybrid packages are transmitted by means of the terminal pins or leads 14.
The largest single cause of hybrid metal packages rejections during visual inspections results from spreading meniscus cracks which exceed fifty percent of the distance from the terminal pin 14 to the eyelet 18 and/or glass chip-out from the meniscus 22.
A typical propagation route 26 for a meniscus crack is shown graphically in FIG. 2. In general, a crack, once formed in the meniscus 22, proceeds inwardly into the glass 12 for some distance, and then at some point hooks back towards the surface of the meniscus 22, such that the crack propagation route 26 is generally "fishhook" in shape, as viewed in cross-section in FIG. 2. The crack propagation route 26 is shown generally as A-B-C-D-E-F-G. A glass chip-out is shown generally at 28 and represents a segment of glass lost from the glass-to-metal seal as a result of a smaller completed fishhook crack.
Cracks or fractures formed in the meniscus, or chip-out losses from the meniscus, may adversely affect the physical integrity of hybrid metal packages. Theoretically, all hermetic packages leak to a certain extent. A "hermetic" package is pragmatically defined as one having an acceptable leak rate, and for most applications the hermeticity is satisfactory if the leak rate is equal to or less than 1.times.10.sup.-8 cubic centimeters of helium per second at a pressure differential of one atmosphere. Cracks or chip-outs in the meniscus may cause the package to have a leak rate greater than 1.times.10.sup.-8 cc/He/sec.
Compression and matched glass-to-metal seals are generally subjected to a salt atmosphere during QC environmental testing. The salt atmosphere will readily penetrate any chip-outs or cracks formed in the meniscus, and if the penetration is sufficient to contact the terminal pin or lead, a corroding action will be engendered thereon. A corroded pin or lead may eventually result in the degradation or complete failure of the hybrid package.
Various solutions have been propounded to reduce the complications arising from meniscus cracking or chip-out loss. These include pressing the glass flat in the meniscus area, treating the glass surface with buffered or unbuffered fluoride ions, undersealing to reduce the degree of meniscus wickup and fire polishing at a slightly lower temperature than the fluidizing temperature during sealing. These solutions not only increase the cost of glass-to-metal seals, but in some cases adversely affect the characteristics of the glass-to-metal seal.
Another approach involves capping the glass preform with a solid ceramic disk. The solid ceramic disk and glass, which in combination comprise the preform, however, have dissimilar CTEs such that when the solid ceramic capped glass preform is subjected to thermal shock or extremes in temperature cycling, the solid ceramic disk and the glass preform expand and contract at different rates due to the differences in CTEs. The disparity in expansion and contraction leads to crack or fracture formation and/or separation along the interface between the solid ceramic disk and glass preform, which makes the glass-to-metal seal generally nonfunctional for its intended purpose.